Convergent beam scanner with multiple mirror defocus compensation

ABSTRACT

Defocus compensation is provided for a convergent beam scanner by using a system of flat scanning mirrors rotating about a spaced parallel common axis instead of single mirror rotating about its central axis.

on 3,885,857 United Stat 1111 3,885,857 Flogaus et al. May 27, 1975 [5 CONVERGENT BEAM SCANNER WITH 2,792,448 5/1957 Deuth et a1. 350/6 MULTIPLE MIRROR DEFOCUS 3.066,576 12/1962 Clorfeine 350/285 3,153,723 10/1964 Weiss 350/7 COMPENSATION 3,614,194 10/1971 Harris 350/7 [75] Inventors: William S. Flogaus, Alexandria;

Mitsumasa Masutani, Ft. Belvoir, GTHER PUBLICATIONS both of Va.

Rutter, Scanning Apparatus, IBM Tech. Disclosure [73] Assignee: The United States of America as Bulletin v 5 N0. 7 19 2 represented by the Secretary of the Army, Washington, DC.

Primary Examzner--R0nald L. W1bert 1 1 Filed: 1973 Assistant Examiner-Michael J. Tokar [21] APPL N0: 408 978 Attorney, Agent, or Firm-Nathan Edelberg; Robert P.

Gibson; John E. Holford [52] US. Cl. 350/7; 250/235; 350/285;

178/16 [57] ABSTRACT [51] Int. Cl. G02b 17/00 [58] Field of Search 350/6, 7, 285, 288, 299; Defocus compensation is provided for a convergent 178/7.6; 250/234-236 beam scanner by using a system of flat scanning mirrors rotating about a spaced parallel common axis in- [56] References Cited stead of single mirror rotating about its central axis.

UNITED STATES PATENTS 6 D 1,136,761 4/1915 Becker ..350/6 rawmg F'gures PATENTEB HAY 2 7 I975 SHEET FIG. 1

DEGREES Af MICROINCHES FIG. 3

PATENTED HAYZTIBYS SHEET CONVERGENT BEAM SCANNER WITH MULTIPLE MIRROR DEFOCUS COMPENSATION The invention described herein may be manufactured. used, and licensed by or for the Government for governmental purposes without payment to us of any royalty thereon.

This application is related in principle to copending application Ser. No. 408,904 entitled Method and Apparatus for Defocus Compensation of a Convergent Beam Scanner" by the same inventors tiled on even date herewith.

BACKGROUND OF INVENTION In the above mentioned copending application a method of compensating for defocus of a convergent beam reflecting off the surface of a rotating plane mirror is described. This was accomplished by rolling the reflecting surface over a cylindrical surface, rather than rotating the mirror about its central axis. A particular type of drive system had to be employed to supply the rocking motion for the mirror as opposed to simple rotary motors previously used. The scanner is intended for use in infrared viewing systems where infrared and visible scanned images are synchronized by reflection on from different sides of the same rotating (or vibrating) scanning mirror. With a simple rotating mirror the processing of the infrared and visible images was achieved by two separate electronic systems on opposite sides of the mirror, so that there was no problem in placement of the elements detectors and light emitting diodes utilized for the two systems. In the single sided scanner of the above mentioned copending application the axis of the infrared objective lens system and axis of the visible eyepiece lens system are parallel and spaced apart less than largest dimension of the small scan mirror. This imposes undesireable limitations on the type of lens systems employed.

SUMMARY OF INVENTION The present invention provides a compensation sys* tem wherein a plurality of plane reflecting mirrors scan a converging or diverging beam directed on or emanating from a linear array of electro-optical elements. The mirrors are balanced around a common axis of rotation thereby permitting the use of a rotary drive motor. The infrared and visible transmitting lens elements are located near opposite ends of the common axis so that they do not interfere with one another. The axis of the lens elements and the common axis are parallel and may be common, if desired.

BRIEF DESCRIPTION OF DRAWINGS The invention is best understood with reference to the attached drawings wherein:

FIG. I shows a first embodiment of the invention having the optical axis of the objective lens and the optical axis oftlfe eyepiece in spaced parallel relationship to the axis of the scanning mirrors;

FIG. 2 shows an exploded view of the internal elements of FIG. I;

FIG. 3 shows a curve of the defocusing (AD as function of scan angle for the device of FIG. I compared to the curve for a similar system using a simple rotating mirror;

FIG. 4 shows an embodiment where the elements of system are arranged in coaxial fashion;

2 FIG. 5 shows an exploded view of the internal elements of FIG. 4; and

FIG. 6 shows a curve of the performance of the FIG. 4 device.

DESCRIPTION OF THE INVENTION In FIG. I the edge view of a generally disc-shaped viewer is shown. A housing member 11 encloses the viewer except for an objective lens 12 and an eyepiece 13 having their optical axes l4 and 15 parallel to the axis 16 ofthe disc-shaped housing II, but spaced therefrom. Such an arrangement allou tdequate room for the internal components.

FIG. 2 shows an exploded view of the internal components. Light energy from a distant image is gathered by the objective lens 12, which may be designed to pass far-infrared energy. A rectangular deflection mirror (or prism) 22 reflects the converging image at right angles as it reaches the approximate center of the axial dimension of the housing. Traveling parallel to the broad circular walls of the housing the beam is again reflected from a first ofa plurality of plane rectangular scanning mirrors 21 having their edges connected and their adjacent plane reflecting surfaces intersecting at equal angles, so that their remaining edges form two regular polygons. The beam finally converges to form a plane image on the surface of a detector 23. The detector surface contains a linear array of electro-optical devices such as photodiodes oriented parallel to the symmetry axis of mirrors 21. A plurality of spoke members 28 extend from the corners of the mirrors to the symmetry axis of the mirrors which axis coincides with the axis 16 of the housing. At this axis the spoke members are rigidly attached to the shaft ofa motor 29, the latter being coaxially attached to the housing 11 from FIG. 1. Members 12, 22 and 23 are also rigidly mounted on the same housing. The detector 23, which includes electronics for amplifying and otherwise processing its output, is coupled by leads 27 to a second linear array of electrooptical devices such as light emitting diodes (LEDs) similar in geometry to the photodiodes on the detector and in the same geometrical relationship to a second mirror, of said mirrors 21, other than the one intercepting the beam to said detector (preferably a diametrically opposite one). A rectangular mirror or prism 26 redirects the beam to the eyepiece 13. The elements are arranged so that, as the mirrors 21 rotate, the entire rectangular image reflected from mirror 22 is uniformly swept over the linear array of photodiodes and the line image produced by the LEDs sweeps out the surface of mirror 26. This is accomplished by proper choice of angular relationships, the radial distance (R) from the axis to mirrors 21 and the spacing (r) of the detector and LEDs from mirrors 21. The geometrical calculations required are easily made by those skilled in the art. The longer the spokes are made the better will be the defocus compensation of this device compared to a similar system using a plane mirror rotating about its central axis. The cross-section of the spokes presented to entering and exiting beams acts as a shutter to remove any stray radiation between scans which might appear in the system. Power for the motor and electronics can come from batteries within the housing or an external source coupled through the housing wall in the usual manner.

A typical system has the following parameters:

relation to the housing. Both mirrors 51 and mirror 52 can be rotated at once, if reversing means such as gears 59 are provided. The necessary electronics to couple F (focal length f icns System, 4 indie the detector and LEDs, a microchip, can be built into E (f/ l m l n pe g) 4 3 5 the mirror. DC power can be supplied from any exter- (spacing of scannlng mirror lrom axis) 2 v (Spacing of Scanning mirror from New, nal source using slip rings. if necessary. or batteries can Foov olfvj c w ll) be mounted in the housing or in tube 56. w (angl e o f iii i crise ilith scan mirror centered) A typlcal FIG 5 device has the following parameters: a (see copending application) Number of Mirrors l2 l Scan Rate (20 lrames/scc) lOO rpm TABLE B f- (focal length of lens system) 6 inches FIG. 3 shows a curve of the defocusing Afat the face F f of lens e 2 of the detector (or the LEDs) as a function of the scan F ig jjfii'g i ijfiiig iiiigl' fgfi fifig H angle of mirrors 2] from the center of each scan. Curve l FOV (Field of View) 7 X 7 A shows the culjve Obtained by a Scan mirror in a Simiifl faii g le d f i rlzi eri e iiiith scan mirror centered) lar system that is merely rotated about its central axis, a: (see copending application) while curve B shows the same values for the system of g g g1 FIGS. 1 and 2. The maximum defocusing can be decan a e ramebsecl rpm creased further by slightly changing the angular relattohshil3 betweeh the mirrors y a Small angle a as FIG. 6 shows the performance curves of the FIG. 5 Plalhed th the above mentioned eopehdthg PP device. Curves A and B have the same relationship as FIG. 4 shows a different embodiment ofthe invention curves A and B in g 3, Th compensation i somewherein the various elements are all coaxial. This does what better than the FIG 2 embodiment and no f h not materially change the total volume required, but correction is indicated Provides a significantly different form faeter- The Obvious many variations of the above structures will Sign Otthe Fitch 1 devtee is based a diameter of more be immediately apparent to those skilled in the art but than twice 1t5 length thickness) While the FIG. 4 the invention islimited only by the claims which follow. embodiment has a length more than twice its diameter. w claim; The objective lens 42 and the eyepiece 43 are compa- 1 An optical system Comprising; Table to elements 12 and 13 in 1 except that y an objective lens means having a first optical axis for are mounted on the axle of housing 41. forming a convergent focused beam of light from FIG. 5 shows an exploded view of the internal elea distant radiating object; ments of The mirrors 51 are arranged in the afirst plane deflection mirror mounted atafirst point Same manner as mirrors 21 in These mirrors on said first axis and inclined thereto to redirect may be mounted for rotation within the housing 21 or id b to a Second point in a reference plane fixed thereto, depending on the mode of operation denormal to said fi Optical axis; sired. Coaxially disposed and within mirrors 51 is a rea i n f piane Scanning mirrors symmetrically directing Both Sides of the mirror are plane 40 mounted in fixed relative positions at a fixed radial reflectors designed to reflect the wavelength associated di ll l to d f i a Second axis, h with the nearest lens, e.g. far-infrared from objective l Center f one of id scanning mirrors i idi 42 and visible for eyepiece 43. The mirror is inclined i h id second reference i to the axis 55 by less than 45 degrees so radiation along a linear array of optical detectors mounted in fixed the axis 55 ill be r ec from it to mirror 51 and 45 relative position to said objective lens and said first back striking the same surface nearer the edge at about d Second points wi h h centers f th i active the same angle ofincidence as the axial reflection. The faces in a first radial plane through said second detector array 53 is mounted on this same surface Of axis, the normal t9 the center of said array passing mirror 52 in the path on the twice reflected beam and through said second reference point making an centered about a plane intersecting the axis 55. The acute angle of at least several degrees with the LED array 54 is placed in a diametrically opposed relastraight line through said first and second points, tionship on the opposite surface of the mirror. Radiasaid acute angle being approximately bisected by tion from this element will obviously be directed to the the normal from said second axis to said second eyepiece according to the same ray geometry Relative point; rotation of the two mirror systems produce essentially 55 a third point located at the center of a second of said the same scanning effect as in the H6. 2 device. There scanning mirrors; will be keystoning and there may be rotation of the an eyepiece lens havingathird optical axis parallel to image at the detector, but these effects will be cansaid first and second axis; celled due to opposite effects produced by the LED 4. a second deflection mirror centered on said third axis It does mean. however, that the maximum scan width and inclined thereto to reflect a light ray from said must be slightly greater than might be expected. lfmirthird point to said third axis; ror 52 is to rotate it can be mounted at the end ofa hola linear array of light emitting elements having their low transparent or windowed tube 56, and the latter centers in a second radial plane through said sec journalled in housing 41 in any convenient manner. ond axis and having a central normal which passes The motor may be located near the wall of housing 41 through said third point. said light emitters being and coupled to the tube 56 as by belt 58. Alternatively mirrors 51 may be edge driven with a belt 57 or ring gear, while mirror 52 is mounted in fixed or rotational electronically coupled to said detector elements, with said lenses. deflection mirrors. and arrays being in fixed relative positions to one another and 6 the spacing of said first and third axes from said 4. A system according to claim 1 wherein said first second axis being equal d e than Said fi and third axes are coaxial with said second axis. dial distance to Said Scanning mirrors; and 5. A system according to claim 1 wherein said arrays means for rotating Said Scanning mirrors relative to are mounted on the reflecting surface ofsaid deflection said relatively fixed elements about said second 5 mirrors.

axis. i

. r co to claim 5 wh 2. A system according to claim 1 wherein said scanj a ac i h d eremd d ning mirrors each are inclined from perpendicular to e ectlon mmor as an secon Oppo the normal to said second axis through said second Parallel reflecnng Surfaces: and point by a small angle a. K) said arrays are mounted on different ones of said sur- 3. A system according to claim 1 wherein said first a tesand third axes are spaced from said second axis. 

1. An optical system comprising: an objective lens means having a first optical axis for forming a convergent focused beam of light from a distant radiating object; a first plane deflection mirror mounted at a first point on said first axis and inclined thereto to redirect said beam to a second point in a reference plane normal to said first optical axis; a plurality of plane scanning mirrors symmetrically mounted in fixed relative positions at a fixed radial distance parallel to and facing a second axis, the center of one of said scanning mirrors coinciding with said second reference point; a linear array of optical detectors mounted in fixed relative position to said objective lens and said first and second points with the centers of their active faces in a first radial plane through said second axis, the normal to the center of said array passing through said second reference point making an acute angle of at least several degrees with the straight line through said first and second points, said acute angle being approximately bisected by the normal from said second axis to said second point; a third point located at the center of a second of said scanning mirrors; an eyepiece lens having a third optical axis parallel to said first and second axis; a second deflection mirror centered on said third axis and inclined thereto to reflect a light ray from said third point to said third axis; a linear array of light emitting elements having their centers in a second radial plane through said second axis and having a central normal which passes through said third point, said light emitters being electronically coupled to said detector elements, with said lenses, deflection mirrors, and arrays being in fixed relative positions to one another and the spacing of said first and third axes from said second axis being equal and less than said fixed radial distance to said scanning mirrors; and means for rotating said scanning mirrors relative to said relatively fixed elements about said second axis.
 2. A system according to claim 1 wherein said scanning mirrors each are inclined from perpendicular to the normal to said second axis through said second point by a small angle Alpha .
 3. A system according to claim 1 wherein said first and third axes are spaced from said second axis.
 4. A system according to claim 1 wherein said first and third axes are coaxial with said second axis.
 5. A system according to claim 1 wherein said arrays are mounted on the reflecting surface of said deflection mirrors.
 6. A system according to claim 5 wherein: said deflection mirror has first and second opposed parallel reflecting surfaces; and said arrays are mounted on different ones of said surfaces. 